MMR
and Autism: Another Study Fails to Find A Link

Question

The parents of a young child are afraid to have
their child immunized due to concerns about the possible role of the
MMR vaccine in causing autism. Does the MMR vaccine cause autism?

Clinical
Bottom Lines

It is unlikely that the MMR vaccine causes autism.

It is impossible to rule out that there may be some unknown subgroup
that is predisposed to autism when exposed to MMR; however such a group
must be very small or the effect must be very minor.

Summary of Key Evidence

This
retrospective cohort study looked at records of all children born in
Denmark from 1991 to 1998 (537,303 kids), of whom 82% received the MMR
vaccine.

Those children who were diagnosed with autism by ICD-10 criteria
(316 with autism, 422 with autistic spectrum) were reported to a central
government registry, which was used in this study. Some of these charts
were randomly reviewed in a blinded manner by a child psychiatrist as
part of the study in order to ensure validity.

Outcomes were presence or absence of diagnosis of autism or autistic
spectrum disorder.

The following subjects were excluded: 45 kids who had a congenital
disorder that has autistic behavior as a component of their disorder,
and 5028 who either died or emigrated.

Method of analysis: Data was broken down into person-years, with
children assigned to non-vaccinated group until the time that they were
vaccinated, if it occurred. Analysis also looked at temporal relationship
of vaccine administration to see if a dose-effect response was seen.
Logistic regression analysis was used.

Applicability: Vaccine strains identical to those used in USA, age
of administration similar to USA, and diagnostic criteria similar to
those used in the USA.

Validity and limitations: There may exist subgroups with predispositions
that have not yet been identified. However, given the results and the
large size of this study, such groups would have to either be small
in size or the predisposition be minor in effect. Even if we feel that
this study was good, we still cannot exclude that MMR vaccine may cause
or contribute to autism in some children. However, we can say that if
such an effect is present, it must be small. Such an effect could be
outweighed by risk of diseases prevented by the vaccine.

Additional
Comments

It has been noted that the incidence of autism is increasing, and
autism tends to appear at 15 months or so, which is the standard timing
of MMR vaccination.

Alleged biological mechanism is that MMR vaccine may cause an autoimmune
reaction that leads to development of autism, but the details of this
alleged mechanism are unknown.

Other studies have been performed to examine this issue, but they
have been of the case-control type or other less-satisfying types of
studies.

The large size and thorough nature of this study was made possible
by the careful record keeping and the socialized nature of the Danish
health care system.

Despite the impressive size and findings of this cohort study, there
are still many who believe that MMR vaccine is responsible for the rise
in the incidence of autism.